Children of moms who reported depressive symptoms during the first nine months after giving birth were more likely to be shorter than their peers by the time they reached preschool age, according to a recent study published in the journal Pediatrics.

Previous research has found that a mother’s postpartum depressive symptoms can influence growth during the first two years of a child’s life, but the new study finds that the effects may persist in even older children.

The researchers examined data on 6,500 kids who were participating in the U.S. Early Childhood Longitudinal Study Birth Cohort from 2001 to 2007. The researchers analyzed the height of children at three different time periods: 9 months, at age 4, and again when they were 5 or 6, approximately kindergarten age.

The research team found that at age 4, kids with mothers who had reported mild or moderate depression when they were infants were 40% more likely to be in the 10th percentile of height or shorter, compared with other kids their age whose mothers did not report early depression symptoms. By age 5, kids of depressed moms were 48% more likely to be at or below the 10th percentile of height.

The study doesn’t prove that mom’s depression causes a child’s short stature, only that the two are associated. And the authors couldn’t confirm the underlying mechanism linking the two factors, but they had some theories: maternal depression can lead to increased stress in kids, for example, and chronically high levels of the stress hormone cortisol have been associated with lower levels of growth hormones in kids. Depressed moms may also have poor feeding practices like spending less time breast feeding.

“Maternal depressive symptoms are associated with insecure attachment in children and with poor parenting behaviors,” the authors write.

The study doesn’t report whether the mothers were suffering from clinically diagnosed postpartum depression, but the Centers for Disease Control and Prevention (CDC) reports that 11% to 18% of women report having frequent postpartum depressive symptoms.

“These children already start with a great disadvantage. What we’re seeing is that there’s not simply a psychological effect, there’s also a physical effect involved here,” Dr. Kenneth Robbins, clinical professor of psychiatry at the University of Wisconsin School of Medicine, who was not associated with the study, told ABC News.

According to the CDC, postpartum depression among women is characterized by the following:

Trouble sleeping when your baby sleeps (more than the lack of sleep typical among new moms)

Feeling numb or disconnected from your baby

Having scary or negative thoughts about the baby; for example, thinking someone will take your baby away or hurt your baby

Worrying that you will hurt the baby

Feeling guilty about not being a good mom, or ashamed that you cannot care for your baby

The CDC recommends that mothers who experience these symptoms consult their doctor or a counselor. It’s important to keep in mind that even the physical process of becoming a mother — such as trying to get pregnant, being pregnant or the birth of the baby — can increase a woman’s risk of feeling depressed.

The study authors agree, and conclude that detecting and treating depressive symptoms in mothers early could prevent further postpartum maternal depression symptoms and subsequent growth delays in preschool and school-aged children.

Looks to me that this study has not isolated time of year of birth as a delivery. I wouldn't be surprised if this is what causes the observed results. Consider:

My $0.02 is that with cutoff ages for school, kids born in Sept tend to be oldest at the beginning of school the school year. If there is any connection to depression, it could be that the seasons affect the mother's mood (Seasonal Affective Disorder) in a skewed way because if you take the average for the 9 months post birth for that sample, it excludes the longest/sunniest days of the year. In other words, the mothers of the OLDEST kids in the grade (born Sept - Dec) have bigger kids to begin with since they're older at the start of the school year - AND the 9 months after giving birth includes the longest/sunniest days of May - Aug but not the darkest. The opposite holds true for the youngest kids, whose mothers endured the winter months but not the summer months after birth. That makes big difference proportionately when the study researches kids that are just 4 or 5 years old!